The present invention relates to an apparatus for treating digested sludge generated when organic waste matter is subjected to anaerobic digestion.
In another aspect, the present invention relates to a method and an apparatus for preventing scale precipitation, particularly MAP scale precipitation, on the inside of a pipe when digested sludge generated by subjecting sludge to anaerobic digestion is transported by the pipe.
In a further aspect, the present invention relates to digested sludge treatment, and more particularly to a treatment method and a treatment apparatus with which phosphorus can be recovered efficiently from digested sludge generated by subjecting sludge to anaerobic digestion.
In a treatment facility for treating organic waste water containing phosphorus and nitrogen, such as sewerage, waste water, and night soil, first the raw sludge (also referred to as primary sludge hereafter) is subjected to solid-liquid separation in a primary sedimentation tank, whereupon the separated supernatant liquid undergoes an activated sludge process to remove organic matter. The activated sludge produced in the activated sludge process is discharged as excess sludge. When raw sludge, excess sludge, or organic waste matter such as night soil or raw refuse is subjected to anaerobic digestion, the organic matter in the waste matter is broken down by the action of acidic bacteria and methanogenic bacteria, leading to a reduction in the amount of sludge, and waste water having high concentrations of methane gas, carbon dioxide, nitrogen, and phosphorus is generated. Today, active investigations are being conducted into methods of utilizing the generated methane gas as a heat source, producing MAP (magnesium ammonium phosphate) from the digested sludge liquor obtained by dewatering the digested sludge, and making effective use of this MAP in fertilizers, chemical raw materials, and so on (Japanese Unexamined Patent Application Publication 2003-117306).
Moreover, anaerobic digestion tanks have recently increased in efficiency in order to save energy and reduce sludge. For example, sludge is solubilized by subjecting raw sludge, excess sludge, or mixed sludge containing both raw sludge and excess sludge to physical/mechanical treatment, chemical liquefaction treatment, heat treatment, and so on, thereby enabling an improvement in the methane gas recovery rate of the subsequent anaerobic digestion process and promoting sludge reduction. Ultrasonic treatment, crushing using a mill, and so on may be cited as examples of such physical/mechanical treatment, while treatment with ozone, hydrogen peroxide, acid, or alkali is included in the chemical liquefaction treatment. Heat treatment includes treatment with thermophilic bacteria and so on Japanese Unexamined Patent Application Publication 2002-336898, for example, discloses a method of solubilizing sludge by treating the sludge in an ultrasonic treatment process.
Nowadays, in order to achieve effective utilization and efficient treatment of digested sludge, regions exist in which all wastewater treatment plants and the like are connected by sewers, and generated digested sludge is transported through the sewers to be treated collectively in a single treatment plant. The construction cost of the sewers is cheaper than that of a treatment facility, and since the advantage of scale applies to sludge treatment facilities (i.e. the unit cost thereof decreases as the scale increases), sewers are considered to be more economical in locations such as urban areas, where residential buildings are in close proximity.
When digested sludge is transported by pipes, so-called MAP precipitate, generated when the magnesium ions of the sludge combine with phosphorus ions and ammonium ions, is generated, leading to possible blockages of the sludge pipe.
To solve this problem, a method of transporting the sludge through a sludge pipe after removing and recovering the MAP by aerating the sludge in a reactor in advance to generate MAP particles, and then subjecting the sludge containing the MAP particles to centrifugal separation, is known.
Furthermore, as the efficiency of anaerobic digestion improves, the nitrogen and phosphorus concentrations of the generated waste water increase. Organic waste matter initially contains elements such as nitrogen, phosphorus, and magnesium, and when the organic waste matter is solubilized, these elements migrate to the liquid. When waste water containing high concentrations of nitrogen and phosphorus is returned to a water treatment system, increased nitrogen and phosphorus loads are applied to the water treatment system, causing a deterioration in the quality of the treated water.
In a known technique for solving this problem, digested sludge or digested sludge liquor is subjected to aeration treatment to generate MAP, and a part of the sludge that is submerged in a sedimentation tank is returned to the aeration tank top serve as seed crystals. In so doing, the phosphorus concentration of the return water decreases, and. MAP recovery is facilitated. In Japanese Examined Patent Application Publication H7-115979, digested sludge is decarbonated, whereupon a magnesium compound is added to precipitate MAP, thereby reducing the phosphorus concentration of liquid. In both cases, the phosphorus concentration of the liquid is decreased by precipitating MAP, and therefore phosphorus can be prevented from circulating endlessly during water treatment and sludge treatment processes.
A method of transporting the sludge through a sludge pipe after removing and recovering MAP by aerating the digested sludge in a reactor in advance to generate MAP particles, and then subjecting the sludge containing the MAP particles to centrifugal separation, is also known. By means of this operation, problems such as blockages of the sludge pipe caused by the MAP particles can be avoided.
As noted above, the nitrogen and phosphorus concentrations of generated waste water increase as the efficiency of anaerobic digestion improves. Organic waste matter initially contains elements such as nitrogen, phosphorus, and magnesium, and when the organic matter is solubilized, these elements migrate into the solution. Nitrogen, phosphorus, and magnesium are constituent components of MAP, and at high concentrations, or when alkali increases, the nitrogen, phosphorus, and magnesium easily equal or exceed the solubility product of the MAP such that the MAP precipitates spontaneously in a digestion tank. When the MAP precipitates onto a draft tube in the digestion tank, the flow of the digested sludge deteriorates, and scale trouble such as blockages during pump extraction occur frequently.
Moreover, the MAP is disposed of together with the dewatered sludge rather than being recovered, and hence there is demand for an efficient MAP recovery method.
When MAP is precipitated by decarbonating the digested sludge or adding a magnesium compound thereto, the phosphorus concentration of the dewatered separated liquid decreases, and therefore the phosphorus load on the water treatment system is reduced so that the treated water can be maintained at a favorable quality. However, this method focuses on phosphorus removal rather than the recovery of phosphorus resources, and hence there is demand for a treatment method which satisfies aspects of both phosphorus removal and phosphorus recovery. Moreover, when MAP is recovered through aeration and centrifugal separation, the recovered substance contains digested sludge and coarse contaminant particles such as night soil residue as well as the MAP, and hence it is not always possible to recover MAP having a high degree of purity. When recycling phosphorus, purity is required, and hence there is demand for a method of recovering MAP having a high degree of purity.
Further, when digested sludge containing MAP is transported by pipe to a facility for treating sludge collectively, a large amount of MAP scale is generated in the pipe, leading to a deterioration in the efficiency with which the sludge is transported. Following its initial generation, MAP scale continues to grow. If MAP scale is left on the inside of the pipe, the entire sewer is eventually covered in MAP scale, making sludge transportation difficult, and as a result, cleaning must be performed periodically and maintenance becomes troublesome.
Furthermore, when MAP is removed and recovered by aerating digested sludge in a reactor in advance to generate MAP particles and then subjecting the sludge containing the MAP particles to centrifugal separation in an attempt to solve the problems described above, coarse contaminant particles such as night soil residue in the digested sludge often cause blockages in the centrifugal separator, particularly when a liquid cyclone is used as the centrifugal separator. As a result, stable treatment is difficult, while cleaning and maintenance are laborious.